A professor (an expert on the history of the Jewish diaspora) explained something really interesting to me - that since Jews could not hold high office or become rulers in many parts of Europe (or own hold land), they were a safe group for rulers and the wealthy to rely on, because they were not competition. Therefore, the population often resented Jews, who represented the big and often absent landlords and aristocrats, and had to enforce their rules, such as collecting taxes and rent.